IPB

Welcome Guest ( Log In | Register )

13 Pages V  < 1 2 3 4 5 > »   
Reply to this topicStart new topic
Post Cook Islands Bedrock and Soil Science Studies, The 3rd leg in our Journey to Endeavour Crater
ElkGroveDan
post Apr 11 2009, 01:20 AM
Post #31


Senior Member
****

Group: Admin
Posts: 4750
Joined: 15-March 05
From: Sloughhouse, CA
Member No.: 197



QUOTE (Vultur @ Apr 10 2009, 02:55 PM) *
Is Oppy going to swerve toward Porcupine? It looks like a very old crater ... have any of the craters so far been quite this old?

Vostok and Erebus which we visited are both really old.


--------------------
If Occam had heard my theory, things would be very different now.
Go to the top of the page
 
+Quote Post
Stu
post Apr 11 2009, 08:36 AM
Post #32


The Poet Dude
****

Group: Moderator
Posts: 5551
Joined: 15-March 04
From: Kendal, Cumbria, UK
Member No.: 60



Some luvverly layers in these rocks...

Attached Image


--------------------
Go to the top of the page
 
+Quote Post
MarkG
post Apr 11 2009, 04:33 PM
Post #33


Member
***

Group: Members
Posts: 145
Joined: 31-October 08
Member No.: 4473



This area seems to be a line/zone of small secondary craters, as has been mentioned before. Is there any discussion of what the primary crater might be? If the primary crater is identified, a rough date determination would be possible.

Is anyone looking into this?

-- MarkG
Go to the top of the page
 
+Quote Post
centsworth_II
post Apr 11 2009, 05:43 PM
Post #34


Senior Member
****

Group: Members
Posts: 2143
Joined: 28-December 04
From: Florida, USA
Member No.: 132



QUOTE (MarkG @ Apr 11 2009, 11:33 AM) *
This area seems to be a line/zone of small secondary craters...

How can you tell secondary craters from a large impact vs. small craters from a fragmented meteor? My inexpert guess is that a small, heavily pocked area indicates the later.
Go to the top of the page
 
+Quote Post
Shaka
post Apr 11 2009, 08:43 PM
Post #35


Senior Member
****

Group: Members
Posts: 1229
Joined: 24-December 05
From: The blue one in between the yellow and red ones.
Member No.: 618



As far as I can tell the freshest crater in the Victoria vicinity is Beagle. It has discernible rays, and its larger ejecta blocks could have flown this far.
Proving a connection between Beagle and any particular pile of rocks is a horse of a different color, however. This nice little Easter Basket of rocks does seem to be the result of a single, large slab smacking down and fragmenting from the shock, followed by wind erosion of the chunks. One could make a stab at estimating the length of time it took for fresh chunks to erode to this condition. That age might then be compared with an estimate for the erosion-age of Beagle, but I don't think I'll try it myself today. rolleyes.gif
Distinguishing eroded secondary craters from clustered primaries is more art than science, but the fresh craters tend to have a deeper profile in the primaries.
If you can line up craters with rays from a primary, you have at least a suggestion that they are secondaries from it. unsure.gif


--------------------
My Grandpa goes to Mars every day and all I get are these lousy T-shirts!
Go to the top of the page
 
+Quote Post
Tesheiner
post Apr 13 2009, 10:27 AM
Post #36


Senior Member
****

Group: Moderator
Posts: 4276
Joined: 19-April 05
From: .br at .es
Member No.: 253



Here's a polar view of the navcams taken at the current site, next to Discovery, during sols 1854/55.
Attached Image
Go to the top of the page
 
+Quote Post
MarkG
post Apr 13 2009, 08:13 PM
Post #37


Member
***

Group: Members
Posts: 145
Joined: 31-October 08
Member No.: 4473



QUOTE (centsworth_II @ Apr 11 2009, 09:43 AM) *
How can you tell secondary craters from a large impact vs. small craters from a fragmented meteor? My inexpert guess is that a small, heavily pocked area indicates the later.


I can try to offer a qualitative answer (I'd need lots of rust-remover to give a quantitative answer...).
The size of a crater is a function of the energy of the impinging object (diam ~ Energy**3 ?). E ~ M*V**2.
So a small object smashing in really fast makes the same size crater as a larger object coming in slower.
There is a dichotomy of approach velocities, with primary objects coming in at or above the Martian escape velocity (5km/sec), and secondary objects being much slower (~2-3 km/sec and under).
Atmospheric drag goes in proportion to the surface area of the impinging object, so smaller objects are more easily slowed down and/or evaporated that large objects. The effect of this is to produce a threshold of size below which crater-producing impacts become unlikely (for given velocity/density/strength/trajectory-angle).
With a large field of meter-ish sized craters, we would expect the group of crater-producing objects to be all within a factor of about 100 in mass (assuming nearly-equal velocities).

Now, I am not sure of the actual numbers, but the Martian atmosphere would essentially filter out smaller impactors. The question to ask here is whether primary impactors that would produce a one-or-two-meter-sized crater are below this threshold, and therefore much less likely?

A primary impactor group would need to split into a number of similar-sized fragments, and the effect of atmospheric drag would tend to scatter their V-squared energy for impact crater size even more.

A large impact producing showers of secondary debris, however, could easily produce a number of similar fragments, since the parent material and the launching shock forces in a given area of the source rock are similar. The ballistic launch of these fragments then has a decent likeleyhood of producing a group of similar-sizes fragments well above the threshold for significant atmospheric drag, but that produce fairly small craters.

(Note that a grazing-impact object might produce a group like this if it breaks up and falls just ballistically...., but that is also a chancey scenario.)

Thus, the secondary impact scenario at this scale seems more probable. Probabilities are the best we can do without samples to analyze. Field trip, anyone?

'Nuff said?

-- MarkG


Go to the top of the page
 
+Quote Post
HughFromAlice
post Apr 13 2009, 10:06 PM
Post #38


Member
***

Group: Members
Posts: 237
Joined: 22-December 07
From: Alice Springs, N.T. Australia
Member No.: 3989



Any news on the dust storms? Couldn't find anything. (PS - Mighty long field trip Mark!)
Go to the top of the page
 
+Quote Post
Deimos
post Apr 14 2009, 12:44 AM
Post #39


Martian Photographer
***

Group: Members
Posts: 324
Joined: 3-March 05
Member No.: 183



See http://www.unmannedspaceflight.com/index.p...st&p=138671. Over the last 2 weeks the dust has continued to fall to just below 1 optical depth at each rover site, but it's been pretty slow and steady. I think there's a press release on the way, but it may have been preempted by Spirit's problems.
Go to the top of the page
 
+Quote Post
helvick
post Apr 14 2009, 01:42 AM
Post #40


Dublin Correspondent
****

Group: Admin
Posts: 1799
Joined: 28-March 05
From: Celbridge, Ireland
Member No.: 220



QUOTE (MarkG @ Apr 13 2009, 08:13 PM) *
Thus, the secondary impact scenario at this scale seems more probable. Probabilities are the best we can do without samples to analyze. Field trip, anyone?

I'm game - where do I sign up? smile.gif If there was time to poke around in some of these smaller craters it might be possible to find enough likely suspects for the impactors to make a call on this but even though I would like to see that done in theory there are far better things to spend the limited rover resources available on.

I ran some back of the envelope numbers on this a long time ago and its pretty clear that despite the fact that the Martian atmosphere is relatively thin it still represents a huge barrier to direct impacts - it's thicker at the ground than Earth's atmosphere is at the altitude where the vast majority of potential terrestrial meteorites get vapourized and by my reckoning you need a fairly large (couple of m diameter, possibly as much as 10) meteor to make it to ground level in one (much ablated) piece. The reality of meteorite impacts on earth shows that smallish fragments do land fairly often, and they do cluster but even so the probability that a given smallish impactor is primary must be small relative to the large amount of secondary debris that goes with each medium\large impact especially given that overall erosion rates are so low.
Go to the top of the page
 
+Quote Post
dvandorn
post Apr 14 2009, 03:23 AM
Post #41


Senior Member
****

Group: Members
Posts: 3419
Joined: 9-February 04
From: Minneapolis, MN, USA
Member No.: 15



However, one of the datasets that is not all that well constrained that impacts on this issue (pardon the pun) is the relative flux of impactors of all sizes at Earth's orbit and at Mars' orbit. I'm not certain it's fair to assume that the flux is constant throughout the inner solar system.

I know that on our Moon, the best way to tell a primary from a secondary is to try and quantify the amount of energy released in the impact event. Without an atmosphere, lunar primary craters retain the glassy impact melt created by high-energy impacts and display a lot of regolith breccia formation, while secondaries don't show the same level of energetic materials transformation.

This kind of thing has long since been eroded away or covered up in most Martian craters, but for the micro-craters it just seems to me there has to be some kind of threshold of size-to-crater that, if primaries, would have to have been formed by really tiny, like smaller-than-sand-grain-sized, impactors... unsure.gif

-the other Doug


--------------------
“The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain
Go to the top of the page
 
+Quote Post
CosmicRocker
post Apr 14 2009, 04:36 AM
Post #42


Senior Member
****

Group: Members
Posts: 2226
Joined: 1-December 04
From: Marble Falls, Texas, USA
Member No.: 116



QUOTE (Tesheiner @ Apr 13 2009, 04:27 AM) *
... at the current site ...
What a beautiful location, perched right at the edge of this unusual crater in the drifts. Thank goodness for MMB and a timely metadata update to carry us to this special place. Thanks also to those individuals and organizations providing the raw imagery. I'm kind of getting accustomed to this armchair planetary exploration.

With atmospheric dust levels falling and the recent, significant cleaning event, I can hardly wait to see where Opportunity will next take us.


--------------------
...Tom

I'm not a Space Fan, I'm a Space Exploration Enthusiast.
Go to the top of the page
 
+Quote Post
centsworth_II
post Apr 14 2009, 07:34 AM
Post #43


Senior Member
****

Group: Members
Posts: 2143
Joined: 28-December 04
From: Florida, USA
Member No.: 132



QUOTE (MarkG @ Apr 13 2009, 04:13 PM) *
I can try to offer a qualitative answer (I'd need lots of rust-remover to give a quantitative answer...).

And my knowledge is purely anecdotal! laugh.gif
I don't mean to challenge your calculations, only to show how a novice observer may see things.

My stew of thoughts (just add salt):

Earth's atmosphere must provide an even better filter for meteors than Mars', and yet we see news reports of fireballs followed by pictures of people holding small fragments that have littered the ground. So I don't have a hard time imagining the same on Mars.

I get the feeling that most or all of the craters in Opportunity's area of Meridiani are very old. They have been buried and exhumed and their ejecta has been weathered to nothing, perhaps by the same steady winds that formed the now still ripples, leaving only berries behind. I don't recall what the informed opinion is on the relative ages of the craters and the ripples.

If it's true that all the nearby largish craters are older than the ripples, then the mini craters in the ripples could not be from their ejecta. I would expect ejecta from craters farther away to be more spread out and not form tight impact clusters.

Anyway that's my two,
centsworth. biggrin.gif
Go to the top of the page
 
+Quote Post
Poolio
post Apr 14 2009, 02:52 PM
Post #44


Member
***

Group: Members
Posts: 128
Joined: 28-October 08
From: Boston, MA
Member No.: 4469



QUOTE (djellison @ Apr 9 2009, 06:35 PM) *
40% - wow! That's got to be up around 500Whrs!

Does anyone know if there was an impact on the Mini-TES? Weren't they going to leave the cover open and hope for a good gust of wind?

EDIT: It's also worth noting, I think, that Oppy's last drive (1856) appears to have been accomplished driving forward.
Go to the top of the page
 
+Quote Post
MarkG
post Apr 15 2009, 12:53 AM
Post #45


Member
***

Group: Members
Posts: 145
Joined: 31-October 08
Member No.: 4473



QUOTE (centsworth_II @ Apr 13 2009, 11:34 PM) *
...
Earth's atmosphere must provide an even better filter for meteors than Mars', and yet we see news reports of fireballs followed by pictures of people holding small fragments that have littered the ground. So I don't have a hard time imagining the same on Mars.
...


If meteors (or their fragments) get slowed down to near-subsonic after hitting the atmosphere, then they just fall like rocks. They hit the ground and maybe bounce and maybe crack or chip the surface, but they don't create an explosive crater. Oppy has already passed by and looked at some of these meteorites laying about on the Meridiani ripples (like the one near the heat shield).

(Well, remembering Oppy passing by that heat shield and meteorite seems like a long time ago... and it was!)


--MarkG
Go to the top of the page
 
+Quote Post

13 Pages V  < 1 2 3 4 5 > » 
Reply to this topicStart new topic

 



RSS Lo-Fi Version Time is now: 19th January 2019 - 07:04 PM
RULES AND GUIDELINES
Please read the Forum Rules and Guidelines before posting.

IMAGE COPYRIGHT
Images posted on UnmannedSpaceflight.com may be copyrighted. Do not reproduce without permission. Read here for further information on space images and copyright.

OPINIONS AND MODERATION
Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators.
SUPPORT THE FORUM
Unmannedspaceflight.com is a project of the Planetary Society and is funded by donations from visitors and members. Help keep this forum up and running by contributing here.